Add option to download Sentinel-1 rho and tau

[scottstanie]

The implementation is a bit hacky- rather than redo the pandas model and geojson data, I'm just swapping the COH12 name in the urls for rho, tau or rmse.

Feel free to close this if you don't have an interested, or if the way it's implemented is ad enough that you'd just redo it.

In [89]: Xrho, prho = get_raster_from_tiles(bounds, tile_shortname='s1_coherence_2020', season='summer', variable='rho')
In [90]: Xtau, ptau  = get_raster_from_tiles(bounds, tile_shortname='s1_coherence_2020', season='summer', variable='tau')

In [104]: plt.figure(); plt.imshow(Xtau[0]/1000.0); plt.title(r"$\tau$"); plt.colorbar()
In [105]: plt.figure(); plt.imshow(Xrho[0]/1000.0); plt.title(r"$\rho$"); plt.colorbar()

[cmarshak]

Thank you, Scott, for adding this. I don't think this that hacky - it's probably better long term to have some of the url manipulation so we are not shipping a ton of tables for every permutation of tile sets (it's also what I do with the numerous Hansen/GLAD datasets).

I am going to merge it - and make some minor modifications:

• remove some of the 3.10+ features because want to use this in the Hyp3 Topsapp plugin (which is still in 3.9)
• remove the self-effacing notes about being hacky - if the library gets too bloated with too many tiles and manipulations, maybe each tile set will have it's own function for validation and such manipulations, which hopefully then can be tested

Thank you again! Happy tiling...